1. Field of the Invention
The present invention relates to television (TV) signal transmission systems in general and in particular to an apparatus and method for transmitting scrambled video signals over a frequency modulated (FM) signal link using a pulse-code modulated frequency shift keyed (FSK) subcarrier in a cable TV (CATV) system.
2. Description of the Related Art
A typical CATV system comprises a headend station and, depending on its size, one or more hub stations for transmitting TV signals to the homes, offices, and the like, of individual subscribers.
A headend station is defined as a source of TV signals. The signals are typically transmitted from the headend station to the hub stations via fiber optic, coaxial and/or microwave transmission lines. The signals are transmitted from a hub station, usually by means of coaxial transmission lines, to the TV sets of individual subscribers and, in the case of large CATV systems, to other hub stations for distribution to its subscribers.
Conventional TV sets are designed to receive amplitude modulated (AM) video signals. The transmission of AM signals, however, suffers from poor performance due to decreasing signal-to-noise ratios which can become unacceptable over long transmission lines, e.g. longer than 5-10 kilometers.
To compensate for the signal-to-noise problem where long transmission lines exist between a headend station and a hub, a common practice has been to transmit FM video signals between the two. As is well known, such signals have a very high signal-to-noise ratio. When received in the hub, the signals are demodulated and then amplitude modulated for transmission to the TV sets connected thereto.
The scrambling of video signals is used in CATV systems to prevent the unauthorized viewing of certain TV programs or channels. While various methods may be used for scrambling a video signal, the most common method currently in use involves suppression of the video sync pulse in an amplitude modulated baseband video signal. In the TV set, sync circuits which are responsive to the sync pulse are used for synchronizing the TV to the transmitted video. If the sync pulse is suppressed, a descrambler is required in order to restore the pulse to the AM signal.
In systems providing scrambled video signals which must be transmitted over long distances, e.g. longer than 5-10 kilometers, FM signal processing has been used in an attempt to compensate for the reduced signal-to-noise ratios encountered. In practice, however, since the resulting FM modulation is actually a modulation within a modulation, the use of FM to compensate for the adverse effects on the signal-to-noise ratio resulting from the AM modulation required for scrambling is found to be inadequate.
In U.S. Pat. No. 4,951,313 there is disclosed a video signal transmission system which takes advantage of FM modulation while avoiding the disadvantages of an amplitude modulation within a frequency modulation by demodulating the AM video signal after it has been scrambled but before it is used to provide a frequency modulated video signal. Specifically, in the patented system, the low frequency components, e.g. 0-50 Hz, which contain the sync pulse information are used to pulse-width modulate a frequency shift keyed (FSK) subcarrier having a nominal frequency of 6.75 MHz. The pulse-width modulated FSK subcarrier and the remainder of the scrambled baseband video signal are then combined and used to provide the frequency modulated video signal. It was found, however, that the use of pulse-width modulation of the subcarrier has led to instabilities in the demodulated video signals under certain circumstances.
Specifically, it was found that phase and amplitude noise in the signal transmission link was being demodulated in the FSK demodulation circuit in the receiver such that there was a superposition of the transmitted signal and the demodulated noise which caused amplitude noise, and jitter in the pulse-width modulated signal transitions at the output of the FSK demodulator. It was also found that the following integrator, i.e. the following RC circuit, used for demodulating the pulse-width modulated signal at the output of the FSK demodulator only partially suppressed the noise and jitter.